1. Field of the Invention
The present invention relates to a truing apparatus for accurately truing a grinding wheel with rounded corners.
2. Description of the Prior Art
There have been used in recent years so-called CBN grinding wheels which use a hard material such as cubic boron nitride as abrasives. Since the CBN grinding wheels are highly expensive, it is necessary in truing them to hold the extent of cutting by a truing tool to a minimum required only for removing wavy irregularities of the grinding wheel so that the grinding wheel will have as long a service life as possible. If the truing tool cut into the grinding wheel for too a small extent, then the wavy irregularities would not completely be removed, resulting in a poor grinding accuracy. Therefore, the truing apparatus for such grinding wheels are required that the extent of cutting by the truing tool be controlled at an accuracy of about .+-. a few .mu.m. In actual truing processes, however, the grinding wheel is thermally expanded due to changes in the ambient temperature and bearing temperature, so that the position of the cylindrical grinding surface varies. The grinding wheel as it is grinded by the truing tool is subjected to small wear which tends to vary from time to time. Accordingly, to enable the truing tool to cut into the grinding wheel with the above accuracy, it is necessary that the truing tool cut to a fixed depth from a reference surface which is the grinding surface of the grinding wheel prior to truing. To this end, the position of the grinding surface being trued has to be accurately measured. One method of such measurement has been to detect the position of the grinding surface with a contact sensor for allowing the truing tool to cut to the desired depth from the grinding surface.
More specifically, grinding wheels having a single grinding surface have conventionally been trued in the following manner.
The truing tool is provided with a contact member movable normally to the grinding wheel. After a tip end of the contact member is aligned with the grinding surface of the truing tool, a truing head supporting the contact member is fed toward the grinding wheel. When a contact between the contact member and the grinding wheel is detected, that is, the grinding surfaces of the truing tool and the grinding wheel are aligned, the truing head is further fed along by a desired interval. Thereafter, the truing head is slid axially toward the grinding wheel to true the latter.
The above practice however has proven unsatisfactory with grinding wheels having end grinding surfaces with rounded corners as well as cylindrical grinding surfaces. More specifically, if the contact member contacted the cylindrical grinding surface normally thereto and detected the postion thereof, the detected signal would contain an error since the rounded corners of the grinding surface are axially displaced due to a thermal expansion of the grinding wheel. Accordingly, the grinding surface of the grinding wheel would not be accurately trued continuously along a desired curved line without compensating for the axial displacement of the rounded corners.